Photodetector strips, for example photodiodes or CCD cells (charge couple device) are used for analysing an image line after line, the image passing in front of the strip. Such an image can for example come from an X-ray non destructive test apparatus for the control of luggage in airport terminals.
The detectors integrate at each moment the light energy projected onto the photosensisitive surface of the detector. The electric output signal of the detector is sampled once per line and is multiplexed with the output signals of the other detectors in order to constitute the signal of one line of the image. In general, the signal output from the detectors is converted into a digital signal in view of a further digital processing.
Because of the integration of the light signal on the photosensisitive surface of the detector, the electric signal is smoothed with respect to the corresponding light signal. The sampling further creates problems of spectral folding when the input light signal has a large bandwidth. The smoothing corresponds to the convolution of the signal by a gate centered on the frequency axis origin; the spectrum of the signal is multiplied by the transfer function of this filter, which is a sinus cardinal.
If the light has a large bandwidth, the transfer function of the filter restricts the bandwidth of the smoothed signal. The effect of the smoothing filter is to enhance the low frequencies. There are thus two phenomena which degrade the information in the useful band of the signal: on the one hand, the signal has been submitted to a low frequency filtering and on the other hand, the folded parts of the spectrum disturb the low frequencies.
A reduction of the size of the photosensible surface of the photodetectors could surely increase the resolution, but would change nothing as to the problem of folding of the spectrum. One could think of making the multiplexed electric signal pass through a filter the transfer function of which is the inversion of that corresponding to the integration of the information on the photosensisitive surface of the detector. But this correction of the spectrum would not eliminate the contributions of the signals due to the folding of the high frequencies.